(select
 (join
  (leftjoin
   (bgp
     (triple ?:a <http://example.org/ns#b> <http://example.org/ns#c>))
   (bgp
     (triple <http://example.org/ns#x> <http://example.org/ns#y>
             <http://example.org/ns#z>)))
  (union
   (bgp
     (triple <http://example.org/ns#x1> <http://example.org/ns#y1>
             <http://example.org/ns#z1>))
   (bgp
     (triple <http://example.org/ns#x2> <http://example.org/ns#y2>
             <http://example.org/ns#z2>))))
 (?:a))